Laterite ores are potentially the world's largest source of nickel and cobalt. In general, most deposits of nickel/cobalt laterites contain three major zones based on lithology, mineralogy and chemical composition. These three zones from the base to the surface, are the saprolite zone, the transition zone and the limonite zone, and generally sit atop weathered bedrock material. There is generally a large variation in total thickness of the laterite deposit, as well as individual zone thickness.
The saprolite zone consists predominantly of “saprolitic serpentine minerals” and a large variety of nickel/magnesium silicate minerals. It contains between 0.5% wt and 4% wt nickel and a high magnesium content, which is normally over 6% wt. The cobalt to nickel weight ratio of saprolite is normally less than 1:10.
The limonite zone, located on the top zone of lateritic ore body, contains nickel ranging from about 0.5% wt to 1.8% wt and consists of goethite-rich, magnetite-rich and/or hematite-rich ore, which is rich in iron, nickel and cobalt content. As it is the top zone, it is subjected to greater weathering and oxidation, which is characterised by a decrease in magnesium and ferrous iron content, and an increase in the ferric iron content. Therefore, it has lower magnesium content than saprolitic type ore. Due to stronger weathering and oxidation, limonitic ore contains dominantly fine and soft particles of goethite and/or hematite. Sometimes weathering and oxidation have not been fully completed and either hematite or goethite rich sections are not present. Alternatively, depending upon the climatic condition, there is formation of clay-type laterites that contain nickel and/or cobalt containing iron/magnesium/aluminium silicates, such as smectite, nontronite and chlorite.
The transition zone is not normally well defined and is composed essentially of limonite and saprolite. It also commonly contains nickel in the range of from 1% wt to 3% wt. with co-existing cobalt ranging from 0.08% wt up to 0.3% wt.
Cobalt existence in zones of saprolite, limonite and transition is generally associated with asbolane, a mineral of hydrated manganese oxide. The cobalt value of a laterite ore deposit is mostly recovered from the limonitic and transition zones.
Although laterite ore deposits are exploitable with surface mining, they have historically been overlooked in favour of underground sulfide deposits as the nickel is readily concentrated by floatation techniques. This is despite the abundant source of nickel bearing laterite ore. Most laterites are generally considered a lower grade of nickel bearing ore for conventional whole ore processing, and more difficult to recover the nickel than from sulfide ores. However, as sulfide ore deposits begin to disappear, lateritic nickel ore deposits are increasingly becoming an important source of nickel and cobalt.
The conventional processes for extracting nickel and cobalt from lateritic ores are generally confined to expensive and/or energy consuming methods. For example, it is known to directly smelt laterite ore, which is quite an energy consuming process. In particular, the saprolitic component may be processed by pyrometallurgical means such as a rotary kiln and electric furnace (RKEF) process to make ferronickel. The limonite component of the laterite ore is generally processed in a hydrometallurgical process, such as a high pressure acid leach (HPAL) process with concentrated sulfuric acid.
Alternative means are being developed such as an atmospheric leach which may take place in agitated vats or tanks. Heap leaching is another method currently being developed for economically extracting metals from ores that may not be suitable for either RKEF, HPAL or indeed atmospheric tank processes.
The leach solution or lixiviant in hydrometallurgical processes for the processing of nickel laterite ores is typically sulfuric acid, although other mineral acids such as hydrochloric or nitric acid are also utilised in certain circumstances. Processes are being developed where the lixiviant includes acid fortified fresh or salinated waters. The pregnant leach solution (PLS) from a sulfuric acid leach from a laterite ore, will produce a sulfate solution that will generally include the desired nickel and cobalt ions together with impurities such as ferrous and ferric ions, aluminium, chromium, manganese and magnesium ions in varying quantities depending upon quality of the PLS and the type of ore being leached.
Nickel and cobalt may be recovered from such solutions by a number of varying techniques. For example, conventional multi-stage neutralisation and sulfidation, ion exchange, solvent extraction, electrowinning and pyrohydrolysis are all well-established techniques in order to recover nickel and cobalt from a nickel PLS solution. Downstream processing in order to recover the nickel is dependent upon adequately removing impurities from the solution.
Because the high iron to nickel concentration ratio in the laterite ore leads to a high iron to nickel concentration ratio in the PLS, the removal of iron with the least nickel and cobalt loss becomes a key step in the recovery of nickel and/or cobalt from such solutions as iron is the most significant impurity in processing nickel laterite ores. Ideally the tailing should be stackable to reduce weight, volume and moisture content.
The major morphological states of iron precipitation are hematite, goethite, hydroxide and jarosite, depending on applied temperature, pH and additives in the iron removal step, for example the presence of alkaline ions will generally result in jarosite precipitation. Hematite and jarosite have higher crystallinity than goethite. Ferric hydroxide is amorphous. As crystallinity of the precipitation product plays a key role in the solid physical properties, for example specific surface area, surface absorption capacity, filter permeability and moisture content, the operational criteria such as the nickel/cobalt loss, solid/liquid separation behaviour and tailings stackability of an iron precipitation depends on its morphology and is generally in the order of hematite, then jarosite, followed by goethite then hydroxide.
In the acid leaching of lateritic ore, the conventional high pressure acid leaching (HPAL) process was developed to dissolve nickel and cobalt and precipitate almost all solubilised ferric iron to insoluble hematite. This was achieved in autoclaves operated at high temperatures (250° C.-300° C.) and associated pressures (around 50 bar). HPAL methods recover high percentages of nickel and cobalt but require expensive, sophisticated equipment to withstand the high pressure and temperature operating conditions.
Alternatives to HPAL processes have been disclosed, for example, tank or vat atmospheric acid leaching (AAL) where the process is generally operated at temperatures to 110° and atmospheric pressure. One such disclosure is U.S. Pat. No. 6,261,527 in the name of BHP Minerals International, Inc., which describes the sequential leaching of limonite and saprolite fractions of laterite ore with sulfuric acid at atmospheric pressure and temperatures below the boiling point, precipitating and discarding most of the dissolved iron as insoluble jarosite solids.
Although jarosite precipitation has good solid/liquid separation behaviour and can trap considerable SO42− from the leachate, the drawback most notably is that it has a low iron content, therefore a high residue weight and volume is generally produced.
There are also serious environmental concerns with the removal of iron as jarosite, as the jarosite compounds are thermodynamically unstable. Jarosite may decompose slowly to iron hydroxides releasing sulfuric acid. The released acid may re-dissolve traces of precipitated heavy metals, such as manganese, chromium, nickel, cobalt, copper and zinc, present in leach residue tailings, thereby mobilising these metals into the underground or surface water around the tailings deposit.
Another disadvantage of jarosite precipitation is that jarosite contains sulfate, and this increases the acid requirement for leaching significantly. Sulfuric acid is usually the single most expensive input in acid leaching processing, so there is also an economic disadvantage in the jarosite process.
Other processes such as Australian patent 2003209829 in the name of BHP Billiton SSM Development Pty Ltd, disclose processes where iron is precipitated as goethite in a sequential atmospheric acid leach (AAL) of limonite and saprolite. Whereas goethite is not as potentially environmentally damaging as jarosite, the mass of iron removal per volume is not as significant as hematite. In addition, goethite also has lower solid/liquid separation behaviour than hematite. This causes a high moisture content leading to difficulties in disposing of the residue.
Australian application 2009201837, also in the name of BHP Billiton SSM Development Pty Ltd, discloses an atmospheric acidic leach (AAL) process where the limonite fraction is processed in a primary leach step, and the saprolite fraction is introduced to the discharge slurry from the primary leach step, together with a hematite seed, in order to initiate hematite precipitation.
There are a number of known techniques to recover nickel and/or cobalt from nickel sulfate solutions. One means in which to recover nickel and/or cobalt from an acidic PLS is with an ion exchange (IX) resin in an IX process. Known ion exchange resins would include a functional group of bis-picolylamine to separate nickel completely from impurities such as ferrous, aluminium, chromium, magnesium and manganese ions and partially from ferric ions. DOWEX M4195™ is a typical resin known for such purposes as it has a higher affinity for nickel ions than ferric ions, but the difference is about two to three folds. If the ferric/nickel concentration ratio in solution is higher or comparable to the resins affinity order, considerable ferric ions are absorbed onto the resin. This leads to a low separation efficiency of nickel over ferric ions.
Another recognised means is by solvent extraction (SX). Commercial reagents such as Cyanex 301™ may be used at low pH. This reagent however also has a high affinity for ferric ions. Given the high affinity for ferric ions, and that the reagent may be degraded by the oxidant ferric ions, it becomes critical to remove ferric ions if this reagent is to be used to recover nickel and/or cobalt.
Laterite ores have a high ferric iron to nickel concentration ratio, and the PLS from an acid leach of a laterite ore will also have a high ferric iron to nickel concentration ratio. As a result, the effective capacity of the resin in an IX process to load nickel, or an organic reagent in an SX process, is reduced, given that it will also load considerable quantities of ferric iron. In an IX process, this leads to a need for high investment in IX resin volume and equipment number to maintain a given nickel production capacity. In addition, the high iron content in the IX eluate increases the reagent consumption needed to recover nickel from the solution, for example, as a nickel hydroxide product.
The present invention aims to overcome or alleviate one or more of the problems associated with prior art processes by developing processes where iron is removed as an impurity as hematite in a process conducted at atmospheric pressure
It is a desired feature of the process that ferric iron is precipitated as hematite in a crystalline form, in an impurity removal step, that makes it more readily disposed of in residue tailings.
In a further desired feature, hematite is precipitated in a crystalline form in an atmospheric pressure step. This has the advantage of avoiding precipitation of iron as unstable jarosite, or goethite which is stable but suffers from relatively high nickel/cobalt loss during precipitation, solid/liquid separation and disposal difficulties due to the amorphous form and high moisture content. Hematite may be precipitated in a more crystalline form, which achieves a compact and “stackable” residue.
It is further a desired feature of the invention to remove ferric ions from a nickel solution prior to any ion exchange process, so as to avoid ferric ions competing with nickel on the resin. Alternatively, it is a desired feature to remove ferric ions from the nickel solution prior to any solvent extraction process, so as to avoid reagent degradation, or iron loading on to the reagent.
A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.